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Q: If a person has a $300,000 401(k) and contributes $22,000 a year for five years, can he double its value in that time period?

A: Another investor looking for the elusive prize in investing: Doubling in five years.

A few weeks ago, another Ask Matt reader asked whether it was possible to double the value of a 401(k) retirement in five years. And the answer is a resounding yes, if you follow the guidance in this previous column.

But this past Ask Matt column dealt with doubling a 401(k) with an existing balance of $20,000, a much smaller value than what you're asking to double. With a 401(k) of the smaller $20,000 size, the value of ongoing contributions from your paycheck are a big factor in determining if you can double in five years.

Now you're asking the question many readers who might be further along in their 401(k) retirement savings what to know: Taking a look at a 401(k) with a larger value of $300,000. What's required to double that in five years?

To find out, let's do the math. Let's first calculate the kind of return that would be needed to double a $300,000 401(k) assuming you're not making any contributions. Running the numbers we learn that you'd need a 14.9% annual return in order to turn your $300,000 401(k) into a $600,000 one in five years.

A 14.9% return might not sound like much following the banner 2010 year for stocks. Including dividends, the broad Standard & Poor's 500 index returned 15.1% in 2010. If those types of returns continued in each of the next five years, you could sit back and watch your $300,000 401(k) climb to a value of $606,035 in five years.

Unfortunately, though, years like 2010 don't repeat all that often. In fact, the long-term average return of large U.S. stocks is 9.4% a year, according to data from IFA.com. At that rate, your $300,000 401(k) would be worth $470,119 in five years. That's a solid 57% return, but far from the 100% needed to double your portfolio.

What's more, depending on how close you are to retirement, it's pretty unrealistic to expect to get 9.4% average annual returns in a retirement portfolio. If you're nearing retirement, you're likely to have a big dose of lower-returning bonds in your portfolio. Assuming you've split your portfolio to be 60% stocks and 40% bonds, you might be more prudent to expect returns closer to 6.5% a year. This rate of return changes everything. At that rate, it would take roughly 12 years to double your $300,000 401(k).

So if you can't count on the markets alone to double the value of your $300,000 401(k), what's the answer? Again, setting up a plan to save money in your 401(k) is what will make your retirement goal possible. You might also need to take on more risk to get a higher market return.

Let's do the math using your assumptions. Starting with a $300,000 401(k) and assuming a 6.5% average annual return, you'd need to contribute $33,190 a year to your 401(k) for it to double in five years. The $22,000 a year annual contribution you're suggesting in your question won't be enough to get you to double the account.

Here's a more realistic forecast. With the $22,000 a year contribution you suggest, plus a 6.5% average annual return, your $300,000 401(k) will be worth $536,286 in five years. That's a solid 78.8% gain, but not a 100% one.

For you to double your 401(k) in five years making $22,000 annual contributions, you'll need an average annual return of 9.3%. To get this return, you'll need to increase your exposure to stocks, and therefore boost your risk.

There are prudent ways to do this, though. For instance, Index Funds Advisors' Portfolio 60 has generated a long-term average annual return of 9.2%, which would have gotten you to your goal. This portfolio calls for a lower weighting in bonds, 30% versus the more common 40%, plus a 7% exposure to riskier emerging markets stocks.

As you can see, it's possible to double even a larger 401(k) in five years making the contribution your suggest. But contributions alone aren't going to make it happen. You'll need greater returns from the markets, which in turn, means you'll need to increase your exposure to stocks. And since stocks are riskier than bonds, going for the double adds to your risk, which is something you must decide for yourself if it's worth the tradeoff.

Matt Krantz is a financial markets reporter at USA TODAY and author of Investing Online for Dummies and Fundamental Analysis for Dummies. He answers a different reader question every weekday in his Ask Matt column at money.usatoday.com. To submit a question, e-mail Matt at mkrantz@usatoday.com. Follow Matt on Twitter at: twitter.com/mattkrantz

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